If you're using Safari, click the download
link. When the widget download is complete, Show Dashboard,
click the Plus sign to display the Widget Bar and click the widget's
icon in the Widget Bar to open it.

If you're using a browser other than
Safari, click the download link. When the widget download is
complete, unarchive it and place it in /Library/Widgets/ in your
home folder. Show Dashboard, click the Plus sign to display the
Widget Bar and click the widget's icon in the Widget Bar to open
it.